This application claims the benefit of a Japanese Patent Application No. 2001-040811 filed Feb. 16, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to signal reproducing methods and storage apparatuses, and more particularly to a signal reproducing method for improving a phase margin of a control system, and to a storage apparatus which employs such a signal reproducing method.
2. Description of the Related Art
In a storage apparatus employing a Redundant Array of Independent Disks (RAID) structure, a plurality of hard disk drives (HDDs) are provided. In such a storage apparatus, a situation frequently occurs where a write is carried out in an on-track state with respect to one HDD and a seek to a target cylinder is carried out with respect to another HDD, at the same timing. In this situation, vibrations of the HDD which is carrying out the seek may affect the HDD which is carrying out the write and cause an off-track state, to thereby deteriorate the performance of the storage apparatus.
Recently, the track per inch (TPI) has increased to increase the recording density of recording media in the storage apparatus. As a result, the off-track state is easily generated even by a small external disturbance. In addition, in order to realize a high-speed access by reducing the access time, a large current is applied in order to accelerate and decelerate a head, thereby increasing vibration applied to the head. Therefore, it is becoming essential to increase the operation band of the control system in order to suppress the external disturbances.
However, in a reproducing system of the storage apparatus, a computation process which generates a signal for controlling the head position inevitably includes a computation (or execution) delay. For this reason, a phase margin of the control system is used up by this computation delay, and there was a first problem in that it is impossible to simply increase the gain of the control system.
On the other hand, when the gain of the control system is simply increased, the phase margin is reduced, and there was a second problem in that a closed loop will oscillate.
In the prior art, there was also a third problem in that, as the velocity of the head increases, it becomes more difficult to quickly and accurately detect the head velocity.